feat(actions): auto-assign milestone

.github/workflows/milestone.yml

@@ -0,0 +1,24 @@
+name: 'Issue/PR Milestones'
+
+on:
+  pull_request:
+    types: [opened, reopened]
+  issues:
+    types: [opened, reopened]
+
+jobs:
+  set:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/github-script@v3
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            const milestone = 2
+            github.issues.update({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              milestone
+            })